Bipolar junction transistor with multiple emitter fingers

ABSTRACT

Methods for fabricating bipolar junction transistors, bipolar junction transistors, and design structures for a bipolar junction transistor. The bipolar junction transistor may include a plurality of emitters that are arranged in distinct emitter fingers. A silicide layer is formed that covers an extrinsic base layer of the bipolar junction transistor and that fills the gaps between adjacent emitters. Non-conductive spacers on the emitter sidewalls electrically insulate the emitters from the silicide layer. The emitters extend through the extrinsic base layer and the silicide layer to contact the intrinsic base layer. The emitters may be formed using sacrificial emitter pedestals in a replacement-type process.

BACKGROUND

The present invention relates to semiconductor device fabrication and,more specifically, to bipolar junction transistors, methods forfabricating bipolar junction transistors, and design structures for abipolar junction transistor.

Bipolar junction transistors are typically found in demanding types ofintegrated circuits, especially integrated circuits for high-frequencyapplications. One high-frequency application for bipolar junctiontransistors is in radiofrequency integrated circuits (RFICs), which areused in wireless communications systems, power amplifiers in cellulartelephones, and other types of high speed integrated circuits. Bipolarjunction transistors may also be combined with complementarymetal-oxide-semiconductor (CMOS) field effect transistors in bipolarcomplementary metal-oxide-semiconductor (BiCMOS) integrated circuits,which take advantage of the positive characteristics of both transistortypes in the construction of the integrated circuit.

Conventional bipolar junction transistors are three-terminal electronicdevices that include three semiconductor regions, namely an emitter, abase, and a collector. Generally, a bipolar junction transistor includesa pair of p-n junctions, namely a collector-base junction and anemitter-base junction. A voltage applied across the emitter-basejunction of a bipolar junction transistor controls the movement ofcharge carriers that produce charge flow between the collector andemitter regions of the bipolar junction transistor.

An NPN bipolar junction transistor includes two regions of n-typesemiconductor material constituting the emitter and collector, and aregion of p-type semiconductor material sandwiched between the tworegions of n-type semiconductor material to constitute the base. A PNPbipolar junction transistor has two regions of p-type semiconductormaterial constituting the emitter and collector, and a region of n-typesemiconductor material sandwiched between two regions of p-typesemiconductor material to constitute the base.

Improved bipolar junction transistors, methods for fabricating bipolarjunction transistors, and design structures for bipolar junctiontransistors are needed that advance the capabilities of the technology.

SUMMARY

According to one embodiment of the present invention, a method isprovided for fabricating a bipolar junction transistor. The methodincludes forming an intrinsic base layer and forming a plurality ofsacrificial emitter pedestals on the intrinsic base layer. An extrinsicbase layer is deposited on the intrinsic base layer in which theextrinsic base layer is in a self-aligned relationship with thesacrificial emitter pedestals. The method further includes forming asilicide layer on the extrinsic base layer, removing the sacrificialemitter pedestals to define emitter windows extending through thesilicide layer and the extrinsic base layer to the intrinsic base layer,and forming a plurality of emitters with one of the emitters disposed ineach of the emitter windows.

According to another embodiment of the present invention, a bipolarjunction transistor includes an intrinsic base layer, a silicide layer,an extrinsic base layer vertically between the intrinsic base layer andthe silicide layer, and a plurality of emitters extending through thesilicide layer and the extrinsic base layer to the intrinsic base layer.

According to another embodiment of the present invention, a hardwaredescription language (HDL) design structure is encoded on amachine-readable data storage medium. The HDL design structure compriseselements that, when processed in a computer-aided design system,generates a machine-executable representation of a bipolar junctiontransistor. The HDL design structure includes an intrinsic base layer,an silicide layer, an extrinsic base layer vertically between theintrinsic base layer and the silicide layer, and a plurality of emittersextending through the silicide layer and the extrinsic base layer to theintrinsic base layer. The HDL design structure may comprise a netlist.The HDL design structure may also reside on storage medium as a dataformat used for the exchange of layout data of integrated circuits. TheHDL design structure may reside in a programmable gate array.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention.

FIGS. 1-8 are cross-sectional views of a portion of a substrate atsuccessive fabrication stages of a processing method for fabricating abipolar junction transistor in accordance with an embodiment of theinvention.

FIG. 9 is a top view of the bipolar junction transistor at thefabrication stage of FIG. 8.

FIG. 10 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test.

DETAILED DESCRIPTION

With reference to FIG. 1 and in accordance with an embodiment of theinvention, a substrate 10 includes trench isolation regions 12 whichcircumscribe and electrically isolate a device region 16. The deviceregion 16 is used in the fabrication of a bipolar junction transistor 92(FIGS. 8, 9).

The substrate 10 may be any type of suitable bulk substrate comprising asemiconductor material suitable for forming an integrated circuit. Forexample, the substrate 10 may be a wafer comprised of a monocrystallinesilicon-containing material, such as single crystal silicon wafer with a(100) crystal lattice orientation. The monocrystalline semiconductormaterial of the substrate 10 may contain a definite defect concentrationand still be considered single crystal. The semiconductor materialcomprising substrate 10 may include an optional epitaxial layer on abulk substrate, such as an epitaxial layer comprised of n-typesemiconductor material that defines a top surface 25 and that covers anoppositely-doped bulk substrate.

The trench isolation regions 12 are formed by a conventional process inthe substrate 10. In one embodiment, the trench isolation regions 12 maybe isolation structures formed by a shallow trench isolation (STI)technique that relies on a lithography and dry etching process thatdefines closed-bottomed trenches in substrate 10, fills the trencheswith dielectric, and planarizes the layer to the top surface 25 of thesubstrate 10 using a chemical mechanical polishing (CMP) process. Thedielectric may be an oxide of silicon, such as densifiedtetraethylorthosilicate (TEOS) deposited by chemical vapor deposition(CVD) or a high-density plasma (HDP) oxide deposited with plasmaassistance.

A collector 18 and subcollector 20 of the bipolar junction transistor 92are present as impurity-doped regions in the device regions 16. Thecollector 18 and subcollector 20 may be formed beneath the top surface25 by introducing an electrically-active dopant, such as an impurityspecies from Group V of the Periodic Table (e.g., phosphorus (P),arsenic (As), or antimony (Sb)) effective to impart an n-typeconductivity in which electrons are the majority carriers and dominatethe electrical conductivity of the host semiconductor material. In oneembodiment, the collector 18 and the subcollector 20 may be formed byseparate ion implantations of n-type impurity species and, thereafter,annealing to activate the impurity species and lessen implantationdamage using techniques and conditions familiar to one skilled in theart. In a specific embodiment, the collector 18 may comprise aselectively implanted collector (SIC) formed by implanting an n-typedopant with selected dose and kinetic energy into the central part ofthe device region 16 and may be formed at any appropriate point in theprocess flow. In a specific embodiment, the subcollector 20 may beformed by a high-current ion implantation followed by lengthy, hightemperature thermal anneal process that dopes a thickness of thesubstrate 10 before the optional epitaxial layer is formed. Duringprocess steps subsequent to implantation, the dopant in the collector 18may diffuse laterally and vertically such that substantially the entirecentral portion of device region 16 becomes doped and is continuousstructurally and electrically with the subcollector 20.

An intrinsic base layer 22, which is comprised of a material suitablefor forming an intrinsic base of the bipolar junction transistor 92, isdeposited as a continuous additive layer on the top surface 25 ofsubstrate 10 and, in particular on the top surface 25 of the deviceregions 16. In the representative embodiment, the intrinsic base layer22 directly contacts the top surface 25 of the device region 16 and atop surface of the trench isolation regions 12 that is coplanar with thetop surface 25. The intrinsic base layer 22 may be comprised of asemiconductor material, such as silicon-germanium (SiGe) includingsilicon (Si) and germanium (Ge) in an alloy with the silicon contentranging from 95 atomic percent to 50 atomic percent and the germaniumcontent ranging from 5 atomic percent to 50 atomic percent. Thegermanium content of the intrinsic base layer 22 may be uniform or thegermanium content of intrinsic base layer 22 may be graded or steppedacross the thickness of intrinsic base layer 22. Alternatively, theintrinsic base layer 22 may be comprised of a different semiconductormaterial, such as silicon (Si). The intrinsic base layer 22 may be dopedwith one or more impurity species, such as boron and/or carbon.

Intrinsic base layer 22 may be formed using a low temperature epitaxial(LTE) growth process (typically at a growth temperature ranging from400° C. to 850° C.). The epitaxial growth process is performed after thetrench isolation regions 12 are formed. The epitaxial growth process maybe non-selective as single crystal semiconductor material (e.g., singlecrystal silicon or SiGe) is deposited epitaxially onto any exposedcrystalline surface such as the exposed top surface 25 of device region16, and non-monocrystalline semiconductor material (e.g., polysilicon orpolycrystalline SiGe) is deposited non-epitaxially onto thenon-crystalline material of the trench isolation regions 12 or regions(not shown) where polycrystalline semiconductor material already exists.

The non-selectivity of the growth process causes the intrinsic baselayer 22 to incorporate topography. Specifically, the intrinsic baselayer 22 includes a raised region 24 above the device region 16, anon-raised region 26 surrounding the raised region 24, and a facetregion 28 between the raised region 24 and the non-raised region 26. Theraised region 24 of the intrinsic base layer 22 is comprised ofmonocrystalline semiconductor material and is laterally positioned invertical alignment with the collector region 18. A top surface of theraised region 24 is elevated relative to a plane containing the topsurface 25 of the device region 16. The raised region 24 iscircumscribed by the shallow trench isolation regions 12.

The non-raised region 26 of the intrinsic base layer 22 is comprised ofpolycrystalline semiconductor material and overlies the trench isolationregions 12 near the raised region 24. In the absence of epitaxialseeding over the trench isolation regions 12, the non-raised region 26forms with a low growth rate outside of the device region 16. The facetregion 28 of the intrinsic base layer 22 may be comprised of a mixtureof polycrystalline and monocrystalline material or comprised ofprimarily single crystal material in facet region 28. The thickness ofthe intrinsic base layer 22 may range from about 10 nm to about 600 nmwith the largest layer thickness in the raised region 24 and the layerthickness of the non-raised region 26 less than the layer thickness ofthe raised region 24. The layer thicknesses herein are evaluated in adirection normal to the top surface 25 of substrate 10.

A base dielectric layer 32 is formed on a top surface 30 of intrinsicbase layer 22 and, in the representative embodiment, directly contactsthe top surface 30. The base dielectric layer 32 reproduces thetopography of the underlying intrinsic base layer 22 in device region16. The base dielectric layer 32 may be an insulating material with adielectric constant (e.g., a permittivity) characteristic of adielectric. In one embodiment, the base dielectric layer 32 may be ahigh temperature oxide (HTO) deposited using rapid thermal process (RTP)at temperatures of 500° C. or higher, and may be comprised of an oxideof silicon, such as SiO₂ having a nominal dielectric constant of 3.9.Alternatively, if the base dielectric layer 32 is comprised of oxide,the material of base dielectric layer 32 may be deposited by a differentdeposition process, by thermal oxidation of silicon (e.g., oxidation athigh pressure with steam (HIPDX)), or by a combination of oxideformation techniques known to those of ordinary skill in the art.

A sacrificial layer stack 31 including sacrificial layers 36, 40 is thenformed. Sacrificial layer 36 is deposited on a top surface 34 of basedielectric layer 32 and directly contacts the top surface 34.Sacrificial layer 40, which is optional, is deposited on a top surface38 of sacrificial layer 36. The sacrificial layers 36, 40 reproduce thetopography of the underlying intrinsic base layer 22.

Sacrificial layer 36 may be comprised of a material with a differentetching selectivity than the material of the underlying base dielectriclayer 32. In one embodiment, sacrificial layer 36 may be comprised ofpolycrystalline silicon (e.g., polysilicon) deposited by a conventionaldeposition process such as low pressure chemical vapor phase deposition(LPCVD) using either silane or disilane as a silicon source or physicalvapor deposition (PVD). Sacrificial layer 40 may be comprised of adielectric material with a different etching selectivity than thematerial of the underlying sacrificial layer 36. In one embodiment,sacrificial layer 40 may be comprised of Si₃N₄ deposited by CVD oranother suitable deposition process.

With reference to FIG. 2 in which like reference numerals refer to likefeatures in FIG. 1 and at a subsequent fabrication stage, thesacrificial layers 36, 40 of the sacrificial layer stack 31 arepatterned using photolithography and etching processes to definesacrificial mandrels in the form of sacrificial emitter pedestals 44,46. To that end, the sacrificial layer stack 31 is masked with apatterned mask layer (not shown). In one embodiment, the mask layer maybe a photoresist layer comprised of a sacrificial organic materialapplied to the top surface 42 of sacrificial layer 40 by spin coatingand pre-baked. The photolithography process entails exposing thephotoresist layer to radiation imaged through a photomask, baking, anddeveloping the resultant latent feature pattern in the exposed resist todefine residual areas of photoresist that mask portions of sacrificiallayer stack 31. In particular, the mask includes resist strips coveringrespective surface areas on a top surface 42 of sacrificial layer 40 atthe intended locations of the sacrificial emitter pedestals 44, 46.

An etching process, such as a reactive-ion etching (RIE) process, isused to remove regions of sacrificial layers 36, 40 not protected by themask layer. For example, an initial segment of the etching process mayremove unprotected regions of sacrificial layer 40 and stop on thematerial of sacrificial layer 36. The etch chemistry may be changed toremove unprotected regions of the underlying sacrificial layer 36 andstop on the material of base dielectric layer 32. Alternatively, asimpler etch chemistry might be used that includes fewer etch steps. Atthe conclusion of the etching process, the top surface 34 of basedielectric layer 32 is exposed aside from the portions of the topsurface 34 covered by the sacrificial emitter pedestals 44, 46.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIG. 2 and at a subsequent fabrication stage, a hardmasklayer 48 is deposited on a top surface 34 of base dielectric layer 32and directly contacts the top surface 34. The hardmask layer 48 may be aconformal blanket layer with a thickness that is independent of thetopography of underlying features, such as the sacrificial emitterpedestals 44, 46. Hardmask layer 48 may be comprised of a dielectricmaterial with a different etching selectivity than the underlying basedielectric layer 32. In one embodiment, hardmask layer 48 may becomprised of silicon nitride (Si₃N₄) deposited using CVD. Alternatively,the material of hardmask layer 48 may be deposited by another suitabledeposition process.

After hardmask layer 48 is deposited, a resist layer 50 comprised of aradiation-sensitive organic material is applied to a top surface 49 ofhardmask layer 48 by spin coating, pre-baked, exposed to radiation toimpart a latent image of a pattern including a window 52 to exposesurface areas spatially registered with the device region 16 for bipolarjunction transistor 92, baked, and then developed with a chemicaldeveloper. Window 52 is defined as an opening in the resist layer 50.

A directional anisotropic etching process like RIE that preferentiallyremoves dielectric material from horizontal surfaces, may be used toremove portions of the hardmask layer 48 in regions unmasked by theresist layer 50. In particular, an opening with an interior edge 47 isdefined in the hardmask layer 48 at the location of window 52 andextends through the hardmask layer 48 to the top surface 30 of intrinsicbase layer 22.

In one embodiment, the etching process is selected with an etchchemistry that selectively removes Si₃N₄ in hardmask layer 48 relativeto SiO₂ in the base dielectric layer 32. The etching process also etchesthe hardmask layer 48 to form non-conductive spacers 56, 58 on thesidewalls of the sacrificial emitter pedestals 44, 46. As best shown inFIG. 9, the non-conductive spacers 56, 58 surround the sidewalls of thesacrificial emitter pedestals 44, 46. Following the etching process, theresist layer 50 is removed by oxygen plasma ashing and/or wet chemicalstripping.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 3 and at a subsequent fabrication stage, the basedielectric layer 32 is removed by an etching process that stops on thematerial constituting intrinsic base layer 22. At the conclusion of theetching process, the top surface 30 of intrinsic base layer 22 isexposed in device region 16. During the etching process, the patternedhardmask layer 48 operates as a hardmask to selectively mask portions ofbase dielectric layer 32 outside of the device region 16. Thesacrificial emitter pedestals 44, 46 and non-conductive spacers 56, 58also respectively mask surface areas of the base dielectric layer 32during the etching process.

At the conclusion of the etching process, portions of the top surface 30of intrinsic base layer 22 inside the interior edge 47 of the opening inthe hardmask layer 48 and not covered by the non-conductive spacers 56,58 and sacrificial emitter pedestals 44, 46 are exposed. These portionsof the top surface 30 are an intended location for the extrinsic baselayer 64 of the bipolar junction transistor 92.

In one embodiment, the etching process may be chemical oxide removal(COR) that removes the material of base dielectric layer 32, ifcomprised of SiO₂, with minimal undercut beneath the non-conductivespacers 56, 58. A COR process utilizes a vapor or, more preferably, amixture flow of hydrogen fluoride (HF) and ammonia (NH₃) in a ratio of1:10 to 10:1 and may be performed at low pressures (e.g., of about 1mTorr to about 100 mTorr) and room temperature. The COR process may beperformed in situ in the deposition chamber or may be performed in anindependent chamber. Sacrificial layer 40 remains unchanged as astructure of the sacrificial layer stack 31 following the etchingprocess. An optional hydrofluoric acid chemical cleaning procedure mayfollow the COR process.

With reference to FIG. 5 in which like reference numerals refer to likefeatures in FIG. 4 and at a subsequent fabrication stage, an extrinsicbase layer 64 is formed on the top surface 30 of intrinsic base layer 22and, in the representative embodiment, directly contacts the top surface30 in device region 16. In one embodiment, the extrinsic base layer 64may be comprised of a semiconductor material (e.g., silicon or SiGe)formed by a selective epitaxial growth (SEG) deposition process. Ifcomprised of SiGe, the concentration of Ge may have a graded or anabrupt profile if the extrinsic base layer 64 is comprised of SiGe, andmay include additional layers, such as a Si cap. Epitaxial growth is aprocess by which a layer of single-crystal material (extrinsic baselayer 64) is deposited on a single-crystal substrate (intrinsic baselayer 22) and in which the crystallographic structure of thesingle-crystal substrate is reproduced in the layer. If the chemicalcomposition of the epitaxial material in the extrinsic base layer 64differs from the chemical composition of the intrinsic base layer 22,then a lattice constant mismatch may be present between the epitaxialmaterial of extrinsic base layer 64 and the intrinsic base layer 22.

In an SEG deposition process, nucleation of the constituentsemiconductor material is suppressed on insulators, such as on the topsurface 49 of the hardmask layer 48 and on the non-conductive spacers56, 58. The selectivity of the SEG deposition process forming theextrinsic base layer 64 may be provided by an etchant, such as hydrogenchloride (HCl), in the reactant stream supplied to the SEG reactionchamber or by the germanium source, such as germane (GeH₄) or digermane(Ge₂H₆), supplied to the SEG reaction chamber. If the extrinsic baselayer 64 does not contain germanium, then a separate etchant may besupplied to the SEG reaction chamber to provide the requisiteselectivity. If the extrinsic base layer 64 contains germanium formedusing a germanium source gas, the provision of an additional etchant tothe SEG reaction chamber is optional.

The extrinsic base layer 64 may be in situ doped during deposition witha concentration of a dopant, such as an impurity species from Group IIIof the Periodic Table (e.g., boron or indium) effective to impart ap-type conductivity in which holes are the majority carriers anddominate the electrical conductivity of the host semiconductor material.The extrinsic base layer 64 may comprise heavily-doped p-typesemiconductor material. The uneven topography of the underlyingintrinsic base layer 22 might be partially reproduced in the extrinsicbase layer 64 on device region 16 so that the extrinsic base layer 64has a raised region 65 that overlies the raised region 24 of theintrinsic base layer 22. In an SEG process, nucleation of theconstituent semiconductor material is suppressed on insulators, such ason the top surface 49 of the hardmask layer 48 or on the non-conductivespacers 56, 58.

The material in the extrinsic base layer 64 is ultimately used to forman extrinsic base of a bipolar junction transistor 92 (FIGS. 8, 9),which is a NPN bipolar junction transistor in the representativeembodiment. During the selective growth process, the semiconductormaterial of the raised region 24 and facet region 28 of intrinsic baselayer 22 operates as a seed crystal or crystalline seed that establishesa crystallographic pattern for the semiconductor material of theextrinsic base layer 64 grown on the raised region 24. Thecrystallographic pattern of the raised region of intrinsic base layer 22is reproduced during selective epitaxy in extrinsic base layer 64 overraised region 24 and facet region 28 so that this region of theextrinsic base layer 64 has approximately the same lattice structure andcrystalline orientation as intrinsic base layer 22 taking into accountany differences in lattice constant from dissimilar materialcompositions.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIG. 5 and at a subsequent fabrication stage, a silicidelayer 70 is formed that consumes part of the extrinsic base layer 64during formation. The remainder of the extrinsic base layer 64 separatesthe silicide layer 70 from the intrinsic base layer 22.

To form silicide layer 70, a conformal layer (not shown) of asilicide-forming metal is formed on a top surface 69 of extrinsic baselayer 64 and over the sacrificial emitter pedestals 44, 46 andnon-conductive spacers 56, 58. The layer of silicide-forming metal maybe a blanket layer deposited by, for example, a CVD process or aphysical vapor deposition (PVD) process. In various embodiments,candidate refractory metals for the silicide-forming metal include, butare not limited to, titanium (Ti), cobalt (Co), or nickel (Ni), each ofwhich has a sintering temperatures of 900° C. or less, exhibitsstability on silicon up to a temperature of 950° C., is characterized bya resistivity in a range of 10 μΩ-cm to 20 μΩ-cm, and is thermallystable during subsequent processing steps. Preferably, the silicide inthe silicide layer 70 represents the lowest resistivity phase of theparticular metal silicide and is thermally stable at temperatures ofsubsequent processing steps. A capping layer (not shown) comprised of ametal nitride, such as titanium nitride (TiN) deposited by directcurrent (DC) sputtering or radio frequency (RF) sputtering, may beapplied to the layer of silicide-forming metal.

A silicidation process is employed that involves one or more annealingsteps to form a silicide phase from the layer of silicide-forming metaland the semiconductor material of the extrinsic base layer 64. Thesilicide phase constituting silicide layer 70 may be characterized as asilicon-germanium silicide contingent upon the composition of extrinsicbase layer 64. In a representative silicidation process, the metal andsemiconductor material (e.g., Si or SiGe) are thermally reacted to anintermediate silicide material. The formation anneal is performedemploying ambients and temperatures well known in the art that cause thereaction between the metal and semiconductor material. For example, thesilicidation process may be conducted in an ambient consisting an inertgas such as nitrogen atmosphere, and by heating the silicide-formingmetal utilizing one or more rapid thermal annealing (RTA) steps at anannealing temperature, which is contingent on the type of silicide, ofabout 400° C. to about 900° C. In silicidation processes, metal-richsilicides initially form and continue to grow until the metal isconsumed. When the metal layer has been consumed, silicides of lowermetal content begin to appear and can continue to grow by consuming themetal-rich silicides.

Following the formation anneal, any silicide-forming metal not convertedinto the silicide of silicide layer 70 and the capping layer may beremoved with, for example, selective wet chemical etch processes. Thewet chemical etch process for removing the unconverted metal mayutilize, for example, a H₂O₂/H₂SO₄ solution or a HNO₃/HCl solution. Thewet chemical etch process for removing the capping layer, if comprisedof TiN, may utilize a HNO₃/HF solution or a mixture of NH₄OH:H₂O₂:H₂O(SCl solution).

After the unconverted metal and capping layer are removed, the silicidelayer 70 may be subjected to another anneal process to form alower-resistance phase for the specific silicide used in silicide layer70. The temperature for the transformation anneal of the silicide layer70 may be higher than the temperature of the formation anneal.

The silicide layer 70 surrounds the non-conductive spacers 56, 58 onsacrificial emitter pedestals 44, 46, which do not support growth of thematerial constituting the silicide layer 70. As a result, the silicidelayer 70 smoothly terminates at the respective perimeters of thenon-conductive spacers 56, 58 and the sacrificial emitter pedestals 44,46 and non-conductive spacers 56, 58 project through and above thesilicide layer 70. These spatial relationships between the silicidelayer 70 and the sacrificial emitter pedestals 44, 46 provide theself-alignment between silicide layer 70 and the emitters 78, 80 (FIGS.8, 9). Because the same structure (i.e., the sacrificial emitterpedestals 44, 46 and non-conductive spacers 56, 58) is used duringself-alignment, the silicide layer 70 and extrinsic base layer 64 areboth self-aligned relative to the emitters 78, 80. In alternativeembodiment, the silicide layer 70 can be formed later in subsequentprocess steps, for example simultaneously with the formation of silicideon the source and drain of CMOS field-effect transistor, which isunderstood by a person having ordinary skill in the art.

With reference to FIG. 7 in which like reference numerals refer to likefeatures in FIG. 6 and at a subsequent fabrication stage, an insulatorlayer 72 is deposited that buries the sacrificial emitter pedestals 44,46. The insulator layer 72 may be comprised of a dielectric, which is aninsulating material having a dielectric constant (e.g., permittivity)characteristic of a dielectric material. In one embodiment, insulatorlayer 72 may be comprised of SiO₂ formed by plasma-enhanced CVD (PECVD)or another suitable deposition process. A top surface 74 of theinsulator layer 72 is planarized using a chemical-mechanical polishing(CMP) process so that the top surface 74 is flat. The CMP processcombines abrasion and dissolution to remove a thickness of the insulatorlayer 72 so that the non-planar topography of the top surface 74 fromthe presence of the sacrificial emitter pedestals 44, 46 is reduced oreliminated, and the top surface 74 is thereby flattened. The CMP processis controlled such that the sacrificial emitter pedestals 44, 46 remainburied beneath the top surface 74 of the insulator layer 72.

With reference to FIGS. 8 and 9 in which like reference numerals referto like features in FIG. 7 and at a subsequent fabrication stage, thetop surface 74 of insulator layer 72 is further recessed relative to thesacrificial emitter pedestals 44, 46 by an etching process, such as RIE.Sacrificial layer 40, which is exposed by the recession of insulatorlayer 72, is then removed from its position between the non-conductivespacers 56, 58. Sacrificial layer 40 may be removed from the sacrificialemitter pedestals 44, 46 using a suitable wet chemical etch or RIE witha suitable etch chemistry.

Sacrificial layer 36, which is exposed after the removal of sacrificiallayer 40, is removed from its position between the non-conductivespacers 56, 58 on the sacrificial emitter pedestals 44, 46. Sacrificiallayer 36 may be etched using dry etching process that removes thematerial of sacrificial layer 36 selective to the materials of basedielectric layer 32, non-conductive spacers 56, 58, and base dielectriclayer 32. The etching process stops upon reaching the top surface 34 ofthe base dielectric layer 32. An etching process such as a hydrofluoricacid type procedure like a dilute hydrofluoric (DHF) or a bufferedhydrofluoric (BHF) wet procedure, or a COR process is then applied toremove portions of the base dielectric layer 32 not covered by thenon-conductive spacers 56, 58.

As a result of the removal of the emitter pedestals 44, 46 from betweenthe non-conductive spacers 56, 58, emitter windows 76, 77 arerespectively formed between the non-conductive spacers 56 and thenon-conductive spacers 58. Each of the emitter windows 76, 77 extendsfrom the apex of the non-conductive spacers 56, 58 to the top surface 30of intrinsic base layer 22.

Emitters 78, 80 of the bipolar junction transistor 92 are respectivelyformed in the emitter windows 76, 77. The non-conductive spacers 56, 58respectively surround the emitters 78, 80 for electrically isolating theemitters 78, 80 from the silicide layer 70. As best shown in FIG. 9,additional emitters 82, 84 are formed in the same manner as emitters 78,80, but at different locations of the device region 16, using areplacement process that relies on distinct sacrificial emitterpedestals similar to sacrificial emitter pedestals 44, 46 and emitterwindows between non-conductive spacers respective similar to emitterwindows 76, 77 and non-conductive spacers 56, 58. Each of the emitters78, 80, 82, 84 contacts, and may directly contact, the raised region 24of intrinsic base layer 22 and, therefore, the intrinsic base of bipolarjunction transistor 92.

The emitters 78, 80, 82, 84 of the bipolar junction transistor 92 may beformed by depositing a layer comprised of a heavily-doped semiconductormaterial and then patterning the deposited layer using lithography andetching processes. For example, the emitters 78, 80, 82, 84 may beformed from polysilicon deposited by CVD or rapid thermal CVD (RTCVD)and heavily doped with a concentration of a dopant, such as animpurities species from Group V of the Periodic Table (e.g., arsenic) toimpart n-type conductivity. The heavy-doping level modifies theresistivity of the polysilicon and may be implemented by in situ dopingthat adds a dopant gas to the CVD reactant gases during the depositionprocess.

The lithography process forming the emitters 78, 80, 82, 84 from thelayer of heavily-doped semiconductor material may utilize photoresistand photolithography to form an etch mask that protects only strips ofthe heavily-doped semiconductor material registered with the respectiveemitter windows, including emitter windows 76, 77, in the representativeembodiment. Alternatively, the etch mask may protect a larger area ofthe heavily-doped semiconductor material to form ‘T′’-shaped emitterheads. An etching process that stops on the material of insulator layer72 is selected to shape the emitters 78, 80 from the protected strips ofheavily-doped semiconductor material. The mask is subsequently stripped,which exposes the top surface 74 of insulator layer 72 surrounding theemitters 78, 80, 82, 84.

The emitters 78, 80, 82, 84 may be arranged in rows and columns todefine emitter fingers 79, 81. Emitter 78, 82 are arranged in oneemitter finger 79, and emitters 80, 84 are arranged in another emitterfinger 81. A gap, G₁, separates the non-conductive spacers 56 on thesidewalls of emitter 78 from the non-conductive spacers on the sidewallsof emitter 82, and separates the non-conductive spacers 58 on thesidewalls of emitter 80 from the non-conductive spacers on the sidewallsof emitter 84. A gap, G₂, separates the non-conductive spacers 56 on thesidewalls of emitter 78 from the non-conductive spacers 58 on thesidewalls of emitter 80, and separates the non-conductive spacers on therespective sidewalls of emitters 82, 84. In alternative embodiments,additional emitter fingers may be added to the device constructionand/or additional emitters can be added to each emitter finger. Each ofthe emitters 78, 80, 82, 84 defines a p-n junction along the interfacewith the intrinsic base layer 22 of the bipolar junction transistor 92.

The emitters 78, 80, 82, 84 may be distributed on the intrinsic baselayer 22 so that the emitter fingers 79, 81 have a parallel alignment.Emitter 78 may be collinear with emitter 82 so that emitter 78 has anend-to-end spatial arrangement with emitter 82 and emitter 80 may becollinear with emitter 84 so that emitter 80 has an end-to-end spatialarrangement with emitter 84. End 130 of emitter 78 and end 132 ofemitter 80 are proximate lengthwise (i.e., in the direction of thelengths of the emitters 78, 80, 82, 84) to end 134 of emitter 82 and end136 of emitter 84, respectively. The emitter fingers 79, 81 arejuxtaposed in a side-by-side arrangement with emitter 78 individuallyjuxtaposed with emitter 80 and emitter 82 individually juxtaposed withemitter 84. Side 140 of emitter 78 and side 142 of emitter 82 areproximate to side 144 of emitter 78 and side 146 of emitter 84,respectively.

The construction of the bipolar junction transistor 92 features multipleemitters 78, 80, 82, 84 in single isolated device region 16. Theextrinsic base layer 64 and, therefore, the extrinsic base of bipolarjunction transistor 92 is raised and comprised of single-crystalsemiconductor material. The silicide layer 70, which fully surrounds theemitters 78, 80, 82, 84, abuts the non-conductive spacers 56, 58 andsimilar non-conductive spacers 57, 59 for emitters 82, 84.

A strip or portion 70 a of the silicide layer 70 is positioned betweenthe adjacent emitter fingers 79, 81 so that the gap, G₂, between theemitter fingers 79, 81 is bridged or spanned by the silicide layer 70.Specifically, the portion 70 a of the silicide layer 70 is partiallypositioned between emitter 78 and emitter 80 and is partially positionedbetween emitter 82 and emitter 84. Another strip or portion 70 b of thesilicide layer 70 is partially positioned between emitter 78 and emitter82 and is partially positioned between emitter 80 and emitter 84 so thatthe gap, G₁, is spanned by the silicide layer 70.

The formation of the non-conductive spacers 56, 58 on the sacrificialemitter pedestals 44, 46 prior to the formation of the extrinsic baselayer 64 and silicide layer 70 may permit the portions 70 a, 70 b of thesilicide layer 70 to directly contact and abut the non-conductivespacers 56, 58. Alternatively, the portions 70 a, 70 b of the silicidelayer 70 may be spaced from the non-conductive spacers 56, 58 in anon-abutting spatial relationship so that the portions 70 a, 70 b do notdirectly contact the non-conductive spacers 56, 58. The portions 70 a,70 b of the silicide layer 70 permits the lateral packing density of theemitters 78, 80, 82, 84 to be increased so that the multiple emitterfingers 79, 81 can be incorporated into the construction of the bipolarjunction transistor 92.

The proximity of the silicide layer 70 to the intrinsic base layer 22also reduces the composite sheet resistance and, thereby, significantlyreduces the base resistance R_(b). The base resistance R_(b) is asignificant parasitic. The reduction in the base resistance may improvethe performance of the bipolar junction transistor 92 by increasingspeed of the device, e.g., an important figure of merit, f_(max), whichis a function of base resistance R_(b).

The bipolar junction transistor 92 possesses a vertical architecture inwhich the emitters 78, 80, 82, 84, intrinsic base layer 22, andcollector 18 of the bipolar junction transistor 92 are verticallyarranged. The intrinsic base layer 22 is located vertically between theemitters 78, 80, 82, 84 and the collector 18. The intrinsic base layer22 defines an intrinsic base of the bipolar junction transistor 92. Theextrinsic base layer 64 defines an extrinsic base of the bipolarjunction transistor 92. One p-n junction is defined at the interfacebetween the emitters 78, 80, 82, 84 and the intrinsic base layer 22.Another p-n junction is defined at the interface between the collector18 and the intrinsic base layer 22.

During the front-end-of-line (FEOL) portion of the fabrication process,the device structure of the bipolar junction transistor 92 may bereplicated across different portions of the surface area of thesubstrate 10. In BiCMOS integrated circuits, complementarymetal-oxide-semiconductor (CMOS) transistors may be formed using otherregions of the substrate 10. As a result, both bipolar and CMOStransistors available on the same substrate 10.

Standard back-end-of-line (BEOL) processing follows, which includesformation of wiring lines and via plugs in dielectric layers to form aninterconnect structure coupled with the bipolar junction transistor 92,as well as other device structures like bipolar junction transistor 92and optionally CMOS transistors (not shown) included in other circuitryfabricated on the substrate 10. Passive circuit elements, such asdiodes, resistors, capacitors, varactors, and inductors, may befabricated in the interconnect structure and available for use in theBiCMOS integrated circuit. As shown in FIG. 9, one or more contacts 88may be formed that supply an electrical contact with the subcollector20, and one or more contacts 90 may be formed that supply an electricalcontact with the extrinsic base and intrinsic base.

FIG. 10 shows a block diagram of an exemplary design flow 100 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 100 includes processes, machines and/ormechanisms for processing design structures or devices to generatelogically or otherwise functionally equivalent representations of thedesign structures and/or devices described above and shown in FIGS. 8,9. The design structures processed and/or generated by design flow 100may be encoded on machine-readable transmission or storage media toinclude data and/or instructions that when executed or otherwiseprocessed on a data processing system generate a logically,structurally, mechanically, or otherwise functionally equivalentrepresentation of hardware components, circuits, devices, or systems.Machines include, but are not limited to, any machine used in an ICdesign process, such as designing, manufacturing, or simulating acircuit, component, device, or system. For example, machines mayinclude: lithography machines, machines and/or equipment for generatingmasks (e.g., e-beam writers), computers or equipment for simulatingdesign structures, any apparatus used in the manufacturing or testprocess, or any machines for programming functionally equivalentrepresentations of the design structures into any medium (e.g., amachine for programming a programmable gate array).

Design flow 100 may vary depending on the type of representation beingdesigned. For example, a design flow 100 for building an applicationspecific IC (ASIC) may differ from a design flow 100 for designing astandard component or from a design flow 100 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 10 illustrates multiple such design structures including an inputdesign structure 102 that is preferably processed by a design process104. Design structure 102 may be a logical simulation design structuregenerated and processed by design process 104 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 102 may also or alternatively comprise data and/or programinstructions that when processed by design process 104, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 102 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 102 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 104 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 8, 9. As such,design structure 102 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 104 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 8, 9 to generate a netlist106 which may contain design structures such as design structure 102.Netlist 106 may comprise, for example, compiled or otherwise processeddata structures representing a list of wires, discrete components, logicgates, control circuits, I/O devices, models, etc. that describes theconnections to other elements and circuits in an integrated circuitdesign. Netlist 106 may be synthesized using an iterative process inwhich netlist 106 is resynthesized one or more times depending on designspecifications and parameters for the device. As with other designstructure types described herein, netlist 106 may be recorded on amachine-readable data storage medium or programmed into a programmablegate array. The medium may be a non-volatile storage medium such as amagnetic or optical disk drive, a programmable gate array, a compactflash, or other flash memory. Additionally, or in the alternative, themedium may be a system or cache memory, buffer space, or electrically oroptically conductive devices and materials on which data packets may betransmitted and intermediately stored via the Internet, or othernetworking suitable means.

Design process 104 may include hardware and software modules forprocessing a variety of input data structure types including netlist106. Such data structure types may reside, for example, within libraryelements 108 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 84 nm, etc.). The data structure types may further include designspecifications 110, characterization data 112, verification data 114,design rules 116, and test data files 118 which may include input testpatterns, output test results, and other testing information. Designprocess 104 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 104 withoutdeviating from the scope and spirit of the invention. Design process 104may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 104 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 102 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 120.Design structure 120 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g., information stored in an IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 102, design structure 120 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 8, 9. In one embodiment, design structure 120may comprise a compiled, executable HDL simulation model thatfunctionally simulates the devices shown in FIGS. 8, 9.

Design structure 120 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.,information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 120 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above and shown in FIGS. 8, 9. Design structure120 may then proceed to a stage 122 where, for example, design structure120: proceeds to tape-out, is released to manufacturing, is released toa mask house, is sent to another design house, is sent back to thecustomer, etc.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to a conventional plane of a semiconductor substrate,regardless of its actual three-dimensional spatial orientation. The term“vertical” refers to a direction perpendicular to the horizontal, asjust defined. The term “lateral” refers to a dimension within thehorizontal plane.

It will be understood that when an element is described as being“connected” or “coupled” to or with another element, it can be directlyconnected or coupled with the other element or, instead, one or moreintervening elements may be present. In contrast, when an element isdescribed as being “directly connected” or “directly coupled” to anotherelement, there are no intervening elements present. When an element isdescribed as being “indirectly connected” or “indirectly coupled” toanother element, there is at least one intervening element present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of fabricating a bipolar junctiontransistor, the method comprising: forming an intrinsic base layer;forming a plurality of sacrificial emitter pedestals on the intrinsicbase layer; depositing an extrinsic base layer on the intrinsic baselayer, the extrinsic base layer in a self-aligned relationship with thesacrificial emitter pedestals; forming a silicide layer on the extrinsicbase layer; removing the sacrificial emitter pedestals to define emitterwindows extending through the silicide layer and the extrinsic baselayer to the intrinsic base layer; and forming a plurality of emitterswith one of the emitters disposed in each of the emitter windows.
 2. Themethod of claim 1 wherein each of the sacrificial emitter pedestalsincludes a plurality of sidewalls, and further comprising: beforedepositing the extrinsic base layer, forming non-conductive spacers onthe sidewalls of each of the sacrificial emitter pedestals.
 3. Themethod of claim 2 wherein forming the plurality of emitters comprises:depositing a layer of a conductive material that fills open spacebetween the non-conductive spacers with a conductor to form theemitters.
 4. The method of claim 2 wherein the non-conductive spacerssurround the sidewalls of the emitters, and each emitter is in acontacting relationship with a top surface of the intrinsic base layer.5. The method of claim 1 wherein the intrinsic base layer is formed on atop surface of a device region, the intrinsic base layer includes araised region that is elevated relative to the top surface of the deviceregion, and the emitters directly contact the top surface on the raisedregion.
 6. The method of claim 5 further comprising: forming shallowtrench isolation regions to define the device region, wherein the raisedregion of the intrinsic base layer is circumscribed by the shallowtrench isolation regions.
 7. The method of claim 1 wherein the emittersare spatially distributed in a first emitter finger and a second emitterfinger, and a first portion of the silicide layer is positioned betweenthe first emitter finger and the second emitter finger.
 8. The method ofclaim 7 wherein the first emitter finger includes a first emitter and asecond emitter having a first end-to-end arrangement, the second emitterfinger includes a third emitter and a fourth emitter having a secondend-to-end arrangement, and the first and second emitter fingers arejuxtaposed in a side-by-side arrangement.
 9. The method of claim 8wherein the first portion of the silicide layer is positioned betweenthe first and third emitters and is positioned between the second andfourth emitters, and the silicide layer includes a second portionpositioned between the first and second emitters and positioned betweenthe third and fourth emitters.
 10. The method of claim 7 wherein each ofthe sacrificial emitter pedestals includes a plurality of sidewalls, andfurther comprising: before depositing the extrinsic base layer, formingnon-conductive spacers on the sidewalls of each of the sacrificialemitter pedestals, wherein the first portion of the silicide layerbridges a gap between the first emitter finger and the second emitterfinger.
 11. The method of claim 10 wherein forming the plurality ofemitters comprises: depositing a layer of a conductive material thatfills open space between the non-conductive spacers with the conductivematerial to form the emitters.
 12. The method of claim 1 wherein theintrinsic base layer is formed on a top surface of a device region, andthe intrinsic base layer is surrounded by an opening in a hardmasklayer.
 13. The method of claim 12 wherein the extrinsic base layer iscomprised of a semiconductor material that is deposited by an epitaxialdeposition process on the intrinsic base layer without depositing thesemiconductor material on the hardmask layer.
 14. The method of claim 12further comprising: implanting an n-type dopant into the device regionto form a collector.
 15. A bipolar junction transistor comprising: anintrinsic base layer; a silicide layer; an extrinsic base layervertically between the intrinsic base layer and the silicide layer; anda plurality of emitters extending through the silicide layer and theextrinsic base layer to the intrinsic base layer.
 16. The bipolarjunction transistor of claim 15 further comprising: a plurality ofnon-conductive spacers, each of the non-conductive spacers laterallydisposed between one of the emitters and the silicide layer.
 17. Thebipolar junction transistor of claim 16 wherein each of thenon-conductive spacers surrounds a respective one of the emitters. 18.The bipolar junction transistor of claim 15 wherein the intrinsic baselayer is formed on a top surface of a device region, the intrinsic baselayer includes a raised region that is elevated relative to the topsurface of the device region, and the emitters directly contact the topsurface on the raised region.
 19. The bipolar junction transistor ofclaim 18 further comprising: a plurality of shallow trench isolationregions defining the device region, the shallow trench isolation regionscircumscribing the raised region of the intrinsic base layer.
 20. Thebipolar junction transistor of claim 18 further comprising: a collectorin the device region and aligned with the raised region of the intrinsicbase layer, the collector comprised of a first semiconductor materialhaving an opposite conductivity type than the intrinsic base layer. 21.The bipolar junction transistor of claim 15 wherein the emitters arespatially distributed in a first emitter finger and a second emitterfinger, and a first portion of the silicide layer is positioned betweenthe first emitter finger and the second emitter finger.
 22. The bipolarjunction transistor of claim 21 wherein the first emitter fingerincludes a first emitter and a second emitter having a first end-to-endarrangement, the second emitter finger includes a third emitter and afourth emitter having a second end-to-end arrangement, and the first andsecond emitter fingers are juxtaposed in a side-by-side arrangement. 23.The bipolar junction transistor of claim 22 wherein the first portion ofthe silicide layer is positioned between the first and third emittersand between the second and fourth emitters, and the silicide layerincludes a second portion positioned between the first and secondemitters and between the third and fourth emitters.
 24. The bipolarjunction transistor of claim 21 further comprising: a plurality ofnon-conductive spacers, each of the non-conductive spacers laterallydisposed between one of the emitters and the silicide layer, wherein thefirst portion of the silicide layer bridges a gap between the firstemitter finger and the second emitter finger.
 25. A hardware descriptionlanguage (HDL) design structure encoded on a machine-readable datastorage medium, the HDL design structure comprising elements that whenprocessed in a computer-aided design system generates amachine-executable representation of a bipolar junction transistor, theHDL design structure comprising: an intrinsic base layer; a silicidelayer; an extrinsic base layer vertically between the silicide layer andthe intrinsic base layer; and a plurality of emitters extending throughthe silicide layer and the extrinsic base layer to the intrinsic baselayer.
 26. The HDL design structure of claim 25 wherein the HDL designstructure comprises a netlist.
 27. The HDL design structure of claim 25wherein the DHL design structure resides on storage medium as a dataformat used for the exchange of layout data of integrated circuits. 28.The HDL design structure of claim 25 wherein the HDL design structureresides in a programmable gate array.